Connecting your PS3 with HD
First you PS3 needs a hdmi cable like this
Plug it into the HDMI port on the back of your PS3 and your HDTV
And does your TV just have just one HD in port?
Connecting PS3 and your TV's HDTV subscription at the same time
If your TV only has one HDMI input and you have some other device plugged, then you'll need a switch that gives you more HDMI ports, like this: